1. The Field of the Invention
The present invention is directed to restoring clarity and optics to plastic surfaces (e.g., aircraft windows). In particular, the present invention is directed to compositions and methods for removing scratches and oxidative damage from plastic surfaces including plastic aircraft windows, plastic covers (e.g., headlight covers), lenses, optical lenses, sighting mechanisms, and other plastic surfaces. Embodiments of the invention further have the ability to prevent further damage due to hazing, scratching, and oxidation so as to extend the life expectancy of such surfaces.
2. The Relevant Technology
Plastic materials have largely replaced glass as an optically transparent and safe covering for most all aircraft windows and canopies due to their light weight, strength, and because the window may comprise a molded polycarbonate plastic that is formed into an aerodynamic shape that fits the profile of the aircraft opening at a fraction of the weight of its glass counterpart. Such plastic materials have also replaced other objects that were once formed of glass. For example, headlight covers of typical late model vehicles are also formed of plastic (e.g., polycarbonate), rather than glass. A typical plastic headlight cover is composed of a molded polycarbonate plastic that is formed into an aerodynamic shape that fits the profile of the front of the automobile.
Plastics are superior to glass in a number of respects. Plastics are lighter than glass while having similar clarity, they are more flexible and able to absorb small impacts, and they are much less likely to shatter in response to large impacts. In addition, plastics can readily be molded into a variety of aerodynamic shapes that are more compatible with modern aircraft and automotive design.
Plastics, however, present a number of disadvantages. For example, they are prone to scratching, hazing, and UV-induced oxidative damage. And while plastic aircraft windows and headlight covers are typically endowed with coatings that protect against scratching and UV damage, such coatings do not provide 100% protection over time. For example, airborne particles, road particles, and other hard, abrasive substances can penetrate the protective coating and cause scratching that degrades the optical properties of the plastic. In addition to physically degrading the optical properties of the plastic surface, scratching leads to a number of other processes that can damage the plastic. For example, scratches (from washing and cleaning the surface) that penetrate the protective coating can allow access to the plastic substrate by exhaust emissions and the chemicals in acid rain. Similarly, scratches that penetrate the protective coating typically present on such surfaces also allow access to the underlying plastic substrate. In addition, heating of the aircraft window by the sun can cause hazing and scratches to expand and contract, allowing greater access to the plastic substrate for UV radiation, oxygen, and/or other environmental constituents. Over time, if these processes are left unchecked, plastic aircraft windows, as well as headlight covers and other plastic surfaces can become hazy and almost opaque, reducing the clarity and optical qualities of the window or cover.
In the case of headlights, the lumens, or light output transmitted through the cover can be greatly reduced. This naturally creates a safety issue due to reduced headlight intensity and effectiveness. For example, reduced headlight intensity can result in an inability to effectively see the road ahead and an inability to be seen by others. For example, an object or person on an unlit road may only be seen at about 50 feet rather than the over 200 feet that would be typical with a new headlight cover, making it difficult to brake in time. In addition, such damaged headlight covers diffuse the light output from the headlight, causing diffused glare to oncoming drivers.
In the case of aircraft windows, it can be difficult for pilots or passengers to see effectively through such damaged plastic windows especially when flying into the sun, creating a safety hazard for pilots as well as crew of military and emergency aircraft.
Replacement is one option. In the case of aircraft, the cost of such aircraft windows can run anywhere from $1,000 to $75,000 per window, which does not account for labor and aircraft down time. Such incidental costs for the military and airlines can be even more significant than the price of the window. Similarly, headlight covers that have been badly damaged by hazing, scratching and/or oxidation can be replaced, but at a relatively high cost. Replacing the plastic covers can be cost prohibitive for many consumers. For example, the typical cost for replacing headlight covers on a car can run anywhere between $300 to $1,500 per headlight, not including installation costs.
There are products that are available that purport to restore plastic surfaces. Most of these products, however, use sand paper, harsh cleaners or processes, cheap waxes, and metal polish intended for other applications. These products can be very detrimental to the fragile clarity and optical characteristics of the plastic surface. Also, whatever benefit is derived from these products is often short lived, as the uncoated plastic surface will quickly become crazed when again exposed to environmental forces.
One example of a product that has been used to remove scratches from plastic surfaces is a system called Micro-Mesh™. Another is available from 3M. Both of these use different varieties of sand paper or sanding discs. The Micro-Mesh™ and 3M systems remove scratches from plastics using a series of rubber-backed sanding cloths or discs with differing sizes of grit. The first series of Micro-Mesh™ steps uses a rubber backed sanding cloth having 8 different grits starting at 2,400 grit. In subsequent steps, sanding cloths of finer grades up to 12,000 gauge grit are used. In addition to using the rubber sanding block, a small amount of antistatic cream is applied after polishing is complete. The 3M system uses sanding discs applied with an air tool followed by an oil based compound or polish. The main problem with these systems is that they both require a very skilled technician to perform the procedure properly.
Such systems have many negative aspects. One negative aspect is that without a skilled technician, the process results in optical distortions because a large amount of plastic must be sanded away in order to remove even the tiniest of scratches. That is, it is necessary to remove enough of the plastic surface to at least equal the depth of the scratch in an even pattern of up, down, and side-to-side motions, known as “cross-hatching”. Once this is done, the optics and clarity of the plastic must be restored, although a major problem is that whenever a sizable area of plastic is removed, a much larger portion of the window surrounding the scratch must also be polished out in order to avoid optical distortion of the plastic in the surrounding area where the scratch was removed. To avoid optical distortion, the user must possess a high level of skill and patience, which requires a high amount of training. Such a high level of training is a practical impossibility with a constant turnover of personnel (e.g., as in the military).
Moreover, repeated scratch removals using this system will greatly reduce the thickness of the plastic and destroy its desired protective properties. In the case of pressurized aircraft, there are minimum thickness requirements for airworthiness. One or perhaps two of these sandings may be sufficient to render the window to thin to meet the standard, requiring expensive replacement. Another negative aspect of existing sanding system is the large amount of time it takes to perform each of the series of sandings for each type of grit. Yet another negative aspect of such systems is that extensive sanding removes any protective UV or other specialty coating from the plastic along with the scratches intended to be removed. Many plastic aircraft windows and lenses include such specialty coatings, which should be preserved if at all possible.
With respect to aircraft windows, another system known as the Plastec EZ Clear system uses an oil based polishing compound to finish the process. Because the polish includes oil, it results in a plastic surface that appears clear and scratch free, but once the window is washed once or twice with soap and water the oil residue is lost, and the scratches and hazing from the grit of the polishing compounds become visible again.
With respect to headlight cover repair systems, many other products use varnishes or clear coatings, such as acrylic spar varnish, to essentially fill in and cover scratches in plastic head light cover. These products are easy and quick to use, but they ultimately do not restore the plastic. A coat of varnish merely covers the scratching and oxidation and does nothing to repair the underlying damage to the plastic. Varnishes and paint generally do not adhere to plastic very well and the varnish is likely to flake off in a short period of time. Moreover, if the refractive index of the varnish coating is dissimilar to that of the underling plastic, each of the filled in scratches will act as a micro lens scattering the light from the headlight. Such a result diffuses the light output so that no real improvement in the problem is achieved. While the headlight may look better after applying the coat of varnish, the varnish will not in fact restore the clarity or optical properties of the cover.
Scratching and oxidation damage to various types of plastic surfaces presents an ongoing problem. In the case of plastic aircraft windows, such damage presents an important safety hazard to pilots and other crew of emergency and military aircraft, and while it may simply be thought of as annoying to passengers on commercial aircraft, a need exists for systems and methods for better restoring such plastic surfaces.